The present invention relates to a brake booster for outputting braking force by increasing pedal pressure by a vacuum force, more particularly to a brake booster for generating braking force greater than braking force controlled by pedal pressure in case of emergency braking.
Conventionally, a braking equipment of a vehicle such as a passenger car comprises a brake booster with a vacuum booster using a vacuum, a master cylinder which is driven by the brake booster to develop brake fluid pressure, wheel cylinders which are supplied with the brake fluid pressure to generate braking force, and an anti-skid brake system (hereinafter, referred to as "ABS") for controlling the brake fluid pressure.
In such a braking equipment, when a driver pedals a brake pedal, the brake fluid pressure developed by the master cylinder is always proportional to the pedal pressure. Accordingly, the braking equipment is not able to generate braking force which is greater than braking force controlled by the pedal pressure by the driver. As a result of this, there is a possibility that the vehicle can not be securely and safely stopped for shortage of braking force when emergency braking is required.
Therefore, a braking equipment, which allows full load operation of the brake booster in case of emergency braking and controls its brake fluid pressure by the ABS so that braking force greater than that controlled by the pedal pressure is outputted during the emergency braking, is proposed in Japanese Unexamined Published Patent Application No. 06-179361.
The braking equipment disclosed in '361 electrically detects the pedaling speed of the brake pedal. When the pedaling speed is greater than a predetermined value, the braking equipment rapidly makes the pressure within a variable pressure chamber to be the atmospheric pressure to allow the full load operation of the brake booster and control the brake fluid pressure by the ABS, thereby securely and safely stopping the vehicle during the emergency braking.
However, in the '361 braking equipment, an electromagnetic valve is used, the stroke of the brake pedal is detected, and an electronic controller controls the electromagnetic valve according to electric detected signal of the pedaling speed, thereby making the structure of the '361 braking equipment complex and the cost higher due to the electronic control devices.